Tag: Cassini space probe

Ever since the Cassini probe arrived at Saturn in 2004, it has revealed some startling things about the planet’s system of moons. Titan, Saturn’s largest moon, has been a particular source of fascination. Between its methane lakes, hydrocarbon-rich atmosphere, and the presence of a “methane cycle” (similar to Earth’s “water cycle”), there is no shortage of fascinating things happening on this Cronian moon.

As if that wasn’t enough, Titan also experiences seasonal changes. At present, winter is beginning in the southern hemisphere, which is characterized by the presence of a strong vortex in the upper atmosphere above the south pole. This represents a reversal of what the Cassini probe witnessed when it first started observing the moon over a decade ago, when similar things were happening in the northern hemisphere.

The large cloud in the stratosphere over Titan’s north pole (left) is similar to Earth’s polar stratospheric clouds (right). Credit: L. NASA/JPL/U. of Ariz./LPGNantes; R. NASA/GSFC/M. Schoeberl

During the course of the meeting, Dr. Athena Coustenis – the Director of Research (1st class) with the National Center for Scientific Research (CNRS) in France – shared the latest atmospheric data retrieved by Cassini. As she stated:

“Cassini’s long mission and frequent visits to Titan have allowed us to observe the pattern of seasonal changes on Titan, in exquisite detail, for the first time. We arrived at the northern mid-winter and have now had the opportunity to monitor Titan’s atmospheric response through two full seasons. Since the equinox, where both hemispheres received equal heating from the Sun, we have seen rapid changes.”

Scientists have been aware of seasonal change on Titan for some time. This is characterized by warm gases rising at the summer pole and cold gases settling down at the winter pole, with heat being circulated through the atmosphere from pole to pole. This cycle experiences periodic reversals as the seasons shift from one hemisphere to the other.

In 2009, Cassini observed a large scale reversal immediately after the equinox of that year. This led to a temperature drop of about 40 °C (104 °F) around the southern polar stratosphere, while the northern hemisphere experienced gradual warming. Within months of the equinox, a trace gas vortex appeared over the south pole that showed glowing patches, while a similar feature disappeared from the north pole.

High in the atmosphere of Titan, large patches of two trace gases glow near the north pole, on the dusk side of the moon, and near the south pole, on the dawn side. Credit: NRAO/AUI/NSF

A reversal like this is significant because it gives astronomers a chance to study Titan’s atmosphere in greater detail. Essentially, the southern polar vortex shows concentrations of trace gases – like complex hydrocarbons, methylacetylne and benzene – which accumulate in the absence of UV light. With winter now upon the southern hemisphere, these gases can be expected to accumulate in abundance.

As Coustenis explained, this is an opportunity for planetary scientists to test out their models for Titan’s atmosphere:

“We’ve had the chance to witness the onset of winter from the beginning and are approaching the peak time for these gas-production processes in the southern hemisphere. We are now looking for new molecules in the atmosphere above Titan’s south polar region that have been predicted by our computer models. Making these detections will help us understand the photochemistry going on.”

Previously, scientists had only been able to observe these gases at high northern latitudes, which persisted well into summer. They were expected to undergo slow photochemical destruction, where exposure to light would break them down depending on their chemical makeup. However, during the past few months, a zone of depleted molecular gas and aerosols has developed at an altitude of between 400 and 500 km across the entire northern hemisphere .

This suggests that, at high altitudes, Titan’s atmosphere has some complex dynamics going on. What these could be is not yet clear, but those who have made the study of Titan’s atmosphere a priority are eager to find out. Between now and the end of Cassini mission (which is slated for Sept. 2017), it is expected that the probe will have provided a complete picture of how Titan’s middle and upper atmospheres behave.

By mission’s end, the Cassini space probe will have conducted more than 100 targeted flybys of Saturn. In so doing, it has effectively witnessed what a full year on Titan looks like, complete with seasonal variability. Not only will this information help us to understand the deeper mysteries of one of the Solar System’s most mysterious moons, it should also come in handy if and when we send astronauts (and maybe even settlers) there someday!

Fans of astronomy are no doubt familiar with the work of Kevin Gill. In the past, he has brought us visualizations of what the Earth would look like if it had a system of rings, what a “Living Mars” would look like – i.e. if it was covered in oceans and lush vegetation – and an artistic rendition of the places we’ve been in our Solar System.

In his latest work, which once again merges the artistic and astronomical, Gill has created a series of images that show Saturn’s moon of Daphnis, and the effect it has on Saturn’s Keeler Gap. Through these images – titled “Daphnis in the Keeler Gap” and “Daphnis and Waves Along the Keeler Gap” – we get to see an artistic rendition of how one of Saturn’s moons interacts with its beautiful ring system.

As one of Saturn’s smallest moons – measuring just 8 km (~5 mi) in diameter – the existence of Daphnis had been previously inferred by astronomers based on the gravitational ripples that were observed on the outer edge of the Keeler Gap. This 42 km (26 mi) wide gap, which lies in Saturn’s A Ring and is approximately 250 km from the its outer edge, is kept clear by Daphnis’ orbit around the planet.

Gill’s rendition of a low-angled look at Saturn’s moon of Daphnis moving through the Keeler Gap. Credit: Kevin GIll/

In 2005, the Cassini space probe finally confirmed the existence of this tiny moon. After analyzing images provided by the probe, the Cassini Imaging Science Team concluded that Daphnis’ path and orbit induce a wavy pattern in the edge of the gap. These waves reach a distance of 1.5 km (0.93 mi) above the ring, due to Daphnis being slightly inclined to the ring’s plane.

However, all the images taken by Cassini showed this effect from a great distance. In order to help people appreciate what it must look like close-up, Gill decided to create the visuals you see here. From his images, the passage of Daphnis is shown to give the A Ring a rippled, wavy appearance. In addition, one can see how Daphnis is inclined slightly above the plane of the A Ring, causing the waves to reach upward.

As Kevin Gill told Universe Today via email, these images were the largely inspired by the most recent images of Saturn’s rings that were provided by Cassini space probe, which returned to an equatorial orbit a few months ago after spending two years in high-inclination orbits:

“These are inspired by a general interest in the moon-ring interactions and some recent Cassini views of Daphnis on the 15th (shown below). This is one of the many aspects of the Saturn system that I imagine would be absolutely breathtaking if you could see it in person and ended up being rather simple to model in Maya.”

Continuing with our “Definitive Guide to Terraforming“, Universe Today is happy to present our guide to terraforming Saturn’s Moons. Beyond the inner Solar System and the Jovian Moons, Saturn has numerous satellites that could be transformed. But should they be?

Around the distant gas giant Saturn lies a system of rings and moons that is unrivaled in terms of beauty. Within this system, there is also enough resources that if humanity were to harness them – i.e. if the issues of transport and infrastructure could be addressed – we would be living in an age a post-scarcity. But on top of that, many of these moons might even be suited to terraforming, where they would be transformed to accommodate human settlers.

As with the case for terraforming Jupiter’s moons, or the terrestrial planets of Mars and Venus, doing so presents many advantages and challenges. At the same time, it presents many moral and ethical dilemmas. And between all of that, terraforming Saturn’s moons would require a massive commitment in time, energy and resources, not to mention reliance on some advanced technologies (some of which haven’t been invented yet).

Titan is a moon shrouded in mystery. Despite multiple flybys and surface exploration conducted in the past few decades, this Cronian moon still manages to surprise us from time to time. In addition to having a dense atmosphere rich in hydrocarbons, which scientists believe may be similar to what Earth’s own atmosphere was like billions of years ago, it appears that methane is to Titan what water is to planet Earth.

In addition, methane fog was also observed by the Cassini space probe back in 2009 as it conducted a flyby of Titan. But recent findings by a team of researchers from York University indicates that the Huygens lander also detected fog during its descent towards the surface in 2005. This evidence, combined with the data obtained by Cassini, have helped to shed light on the weather patterns of this mysterious moon.

Thanks the Voyager missions and the more recent flybys conducted by the Cassini space probe, Saturn’s system of moons have become a major source of interest for scientists and astronomers. From water ice and interior oceans, to some interesting surface features caused by impact craters and geological forces, Saturn’s moons have proven to be a treasure trove of discoveries.

This is particularly true of Saturn’s moon Tethys, also known as a “Death Star Moon” (because of the massive crater that marks its surface). In addition to closely resembling the space station out of Star Wars lore, it boasts the largest valleys in the Solar System and is composed mainly of water ice. In addition, it has much in common with two of its Cronian peers, Mimas and Rhea, which also resemble a certain moon-size space station.

Discovery and Naming:
Originally discovered by Giovanni Cassini in 1684, Tethys is one of four moons discovered by the great Italian mathematician, astronomer, astrologer and engineer between the years of 1671 and 1684. These include Rhea and Iapetus, which he discovered in 1671-72; and Dione, which he discovered alongside Tethys.

Cassini observed all of these moons using a large aerial telescope he set up on the grounds of the Paris Observatory. At the time of their discovery, he named the four new moons “Sider Lodoicea” (“the stars of Louis”) in honor of his patron, king Louis XIV of France.

An engraving of the Paris Observatory during Cassini’s time. Credit: Public Domain

Size, Mass and Orbit:With a mean radius of 531.1 ± 0.6 km and a mass of 6.1745 ×1020 kg, Tethys is equivalent in size to 0.083 Earths and 0.000103 times as massive. Its size and mass also mean that it is the 16th-largest moon in the Solar System, and more massive than all known moons smaller than itself combined. At an average distance (semi-major axis) of 294,619 km, Tethys is the third furthest large moon from Saturn and the 13th most distant moon over all.

Tethys’ has virtually no orbital eccentricity, but it does have an orbital inclination of about 1°. This means that the moon is locked in an inclination resonance with Saturn’s moon Mimas, though this does not cause any noticeable orbital eccentricity or tidal heating. Tethys has two co-orbital moons, Telesto and Calypso, which orbit near Tethys’s Lagrange Points.

Tethys’ orbit lies deep inside the magnetosphere of Saturn, which means that the plasma co-rotating with the planet strikes the trailing hemisphere of the moon. Tethys is also subject to constant bombardment by the energetic particles (electrons and ions) present in the magnetosphere.

Composition and Surface Features:Tethys has a mean density of 0.984 ± 0.003 grams per cubic centimeter. Since water is 1 g/cm3, this means that Tethys is comprised almost entirely of water ice. In essence, if the moon were brought closer to the Sun, the vast majority of the moon would sublimate and evaporate away.

It is not currently known whether Tethys is differentiated into a rocky core and ice mantle. However, given the fact that rock accounts for less 6% of its mass, a differentiated Tethys would have a core that did not exceed 145 km in radius. On the other hand, Tethys’ shape – which resembles that of a triaxial ellipsoid – is consistent with it having a homogeneous interior (i.e. a mix of ice and rock).

This ice is also very reflective, which makes Tethys the second-brightest of the moons of Saturn, after Enceladus. There are two different regions of terrain on Tethys. One portion is ancient, with densely packed craters, while the other parts are darker and have less cratering. The surface is also marked by numerous large faults or graben.

The western hemisphere of Tethys is dominated by a huge, shallow crater called Odysseus. At 400 km across, it is the largest crater on the surface, and roughly 2/5th the size of Tethys itself. Due to its position, shape, and the fact that a section in the middle is raised, this crater is also responsible for lending the moon it’s “Death Star” appearance.

The largest graben, Ithaca Chasma, is about 100 km wide and more than 2000 km long, making it the second longest valley in the Solar System. Named after the island of Ithaca in Greece, this valley runs approximately three-quarters of the way around Tethys’ circumference. It is also approximately concentric with Odysseus crater, which has led some astronomers to theorize that the two features might be related.

Scientists also think that Tethys was once internally active and that cryovolcanism led to endogenous resurfacing and surface renewal. This is due to the fact that a small part of the surface is covered by smooth plains, which are devoid of the craters and graben that cover much of the planet. The most likely explanation is that subsurface volcanoes deposited fresh material on the surface and smoothed out its features.

Cassini closeup of the southern end of Ithaca Chasma. Credit: NASA/JPL/Space Science Institute.

Like all other regular moons of Saturn, Tethys is believed to have formed from the Saturnian sub-nebula – a disk of gas and dust that surrounded Saturn soon after its formation. As this dust and gas coalesced, it formed Tethys and its two co-orbital moons: Telesto and Calypso. Hence why these two moons were captured into Tethys’ Lagrangian points, with one orbiting ahead of Tethys and the other following behind.

Exploration:Tethys has been approached by several space probes in the past, including Pioneer 11 (1979), Voyager 1 (1980) and Voyager 2 (1981). Although both Voyager spacecraft took images of the surface, only those taken by Voyager 2 were of high enough resolution to truly map the surface. While Voyager 1 managed to capture an image of Ithaca Chasma, it was the Voyager 2 mission that revealed much about the surface and imaged the Odysseus crater.

Tethys has also been photographed multiple times by the Cassini orbiter since 2004. By 2014, all of the images taken by Cassini allowed for a series of enhanced-color maps that detailed the surface of the entire planet (shown below). The color and brightness of Tethys’ surface have since become sources of interest to astronomers.

On the leading hemisphere of the moon, spacecraft have found a dark bluish band spanning 20° to the south and north from the equator. The band has an elliptical shape getting narrower as it approaches the trailing hemisphere, which is similar to the one found on Mimas.

Global, color mosaics of Saturn’s moon Tethys, as produced from images taken by NASA’s Cassini spacecraft between 2004-2014. Credit: NASA/JPL-Caltech/Space Science Institute/ Lunar and Planetary Institute

The band is likely caused by the influence of energetic electrons from Saturn’s magnetosphere, which drift in the direction opposite to the rotation of the planet and impact areas on the leading hemisphere close to the equator. Temperature maps of Tethys obtained by Cassini have shown this bluish region to be cooler at midday than surrounding areas.

At present, Tethys’ water-rich composition remains unexplained. One of the most interesting explanations proposed is that the rings and inner moons accreted from the ice-rich crust of a much larger, Titan-sized moon before it was swallowed up by Saturn. This, and other mysteries, will likely be addressed by future space probe missions.

We have many great articles about Tethys here at Universe Today. Here’s one about the story about Tethys, with a photograph taken by NASA’s Cassini spacecraft, and another about a feature on the surface of Tethys called Ithaca Chasma.

During its 2006 flyby of Titan, the Cassini Space Probe captured some of the most detailed images of Saturn’s largest moon. Amongst them was one showing the lofty cloud formations over Titan’s north pole (shown above). Interestingly enough, these cloud formations bear a strong resemblance to those that are seen in Earth’s own polar stratosphere.

However, unlike Earth’s, these clouds are composed entirely of liquid methane and ethane. Given Titan’s incredibly low temperatures – minus 185 °C (-300 °F) – it’s not surprising that such a dense atmosphere of liquid hydrocarbons exists, or that seas of methane cover the planet.

What is surprising, however, is the fact that methane crystals also exist in this atmosphere. Eight years after the photos of Titan’s north pole were taken, astronomers have concluded that this region also contains trace amounts of methane ice.

“The idea that methane clouds could form this high on Titan is completely new,” said Carrie Anderson, a Cassini participating scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, and lead author of the study. “Nobody considered that possible before.”

Other stratospheric clouds had been identified on Titan already, including clouds of ethane – a chemical formed after methane breaks down. Delicate clouds of cyanoacetylene and hydrogen cyanide, which form from reactions of methane byproducts with nitrogen molecules, have also been found there.

But clouds of frozen methane were thought unlikely in Titan’s stratosphere. Because the troposphere traps most of the moisture, stratospheric clouds require extreme cold. Even the stratosphere temperature of minus 203 °C (-333 °F), observed by Cassini just south of the equator, was not cold enough to allow the scant methane in this region of the atmosphere to condense into ice.

What Anderson and her Goddard co-author, Robert Samuelson, noted is that temperatures in Titan’s lower stratosphere are not the same at all latitudes. This was based on data taken from Cassini’s Composite Infrared Spectrometer and the spacecraft’s radio science instrument, which showed that the high-altitude temperature near the north pole was much colder than that just south of the equator.

It turns out that this temperature difference – as much as 6 °C (11 °F) – is more than enough to yield methane ice.

Other observations made of Titan’s cloud system support this conclusion, such as how certain regions appear denser than others, and the larger particles detected are the right size for methane ice. They also confirmed that the expected amount of methane – 1.5%, which is enough to form ice particles – is present in the lower polar stratosphere.

What’s more, the observation confirms certain models of how Titan’s atmosphere is thought to work.

According to this model, Titan has a global circulation pattern in which warm air in the summer hemisphere wells up from the surface and enters the stratosphere, slowly making its way to the winter pole. There, the air mass sinks back down, cooling as it descends, which allows the stratospheric methane clouds to form.

“Cassini has been steadily gathering evidence of this global circulation pattern, and the identification of this new methane cloud is another strong indicator that the process works the way we think it does,” said Michael Flasar, Goddard scientist and principal investigator for Cassini’s Composite Infrared Spectrometer (CIRS).

Like Earth’s stratospheric clouds, Titan’s methane cloud was located near the winter pole, above 65 degrees north latitude. Anderson and Samuelson estimate that this type of cloud system – which they call subsidence-induced methane clouds (or SIMCs for short) – could develop between 30,000 to 50,000 meters (98,000 to 164,000 feet) in altitude above Titan’s surface.

“Titan continues to amaze with natural processes similar to those on the Earth, yet involving materials different from our familiar water,” said Scott Edgington, Cassini deputy project scientist at NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, California. “As we approach southern winter solstice on Titan, we will further explore how these cloud formation processes might vary with season.”

The results of this study are available online in the November issue of Icarus.

Ever since the Cassini space probe conducted its first flyby of Enceladus in 2005, the strange Saturnian moon has provided us with a treasure trove of images and scientific wonders. These include the jets of icy water vapor periodically bursting from its south pole, the possibility of an interior ocean – which may even harbor life – and the strange green-blue stripes located around the south pole.

These stripes are essentially four fractures bounded on either side by ridges that appear to be composed of mint-green-colored ice. Known unofficially as “tiger stripes”, these surface fractures have become a source of interest for astronomers since they appear to be the youngest features in the region.

Recently, between these stripes, over 100,000 ice blocks were observed, and they are a further source of wonder. Scientists with the Division of Geological and Planetary Sciences at the California Institute of Technology were able to map out the locations of these blocks in the hopes of determining just how they got there.

Their findings, which are scheduled to appear in the January 2015 issue of Icarus (vol. 245), constitute the first quantitative estimates of ice-block number and density in Enceladus’ southern polar region in relation to major geological features.

Elevated View of Enceladus’ South Pole. Credit: NASA/JPL

The preliminary results of their work reveal that ice blocks in the southern hemisphere are most concentrated within the geologically active South Polar Terrain (SPT) and chiefly concentrated within 20 km of the tiger-stripe fractures. They found further that the ice blocks are concentrated just as heavily between tiger-stripe fractures as on the directly adjacent margins.

To ascertain just how these ice-blocks formed and evolved, and how they came to be distributed in the southern region, the team considered various mechanisms. These included the well-known aspects and features of the moon – namely its seismic activity, impacts by meteors, and volcanic eruptions – but also the possible roles of tectonic disruption of the icy surface mantle and ice slides.

Ultimately, they concluded that impact cratering as well as slides, perhaps triggered by seismic events, could account for a majority of ice-block features within the inner SPT.

However, they also noted that cryovolcanic activity – i.e., the ejection of icy material caused by sub-surface volcanic eruption, and the condensation of ice around the eruption vents – could not be ruled out.

They noted that the pervasiveness of fracturing on many size scales, the sheer number of ice blocks in the inner SPT, and the occurrence of linear block arrangements that parallel crack networks along the flanks of tiger stripes, would seem to indicate that tectonic deformation also played an important role.

Furthermore, they postulated that nearer to the warm tiger-stripe fractures, sublimation likely leads to erosion and disaggregation, which plays a role as well.

Last, they noted that the relative scarcity of blocks beyond the bounds of the SPT, particularly on old, cratered terrains, may be attributed to ice grains accumulating on the surface over time rather than the same causal factors that led to the 100,000 blocks observed around in the southern region.

In short, the CIT team believes that the unusual ice-block formation around Enceladus’ south pole is chiefly the result of impacts from meteors or comets and seismic activity, but that the peculiar activity in this young region of the planet – such as volcanic eruptions from the hypothesized interior-ocean – may also play a role.

The SPT ice-blocks were observed at very high resolution during Cassini’s July 14th flyby, when it observed the “blue ice” tiger stripe” around the south pole and noticed an area of extreme tectonic deformation. The blocks were manually identified and mapped from twenty of the highest resolution photos taken by Cassini’s Imaging Science Subsystem (ISS) and rendered using ArcGIS software.

Cassini continues to study Enceladus, and in fact the spacecraft conducted its latest flyby of Enceladus today at 15:23 (03:23 pm UTC) and its next scheduled flyby will be taking place on Dec. 19th, 2015, at 17:49 (05:49 pm UTC).